This study explores novel methods to assess how individuals interact within social networks. Communication technologies such as cell phones have made it easier to maintain and develop network ties. The nature of how information, norms, and influence flow through networks using these technologies and how it might relate to network characteristics and risk within networks is not well known. This proposal explores novel and innovative methods of assessing interactions within social networks and their impact on risk behavior (e.g., sexual risk, substance use) among emerging adult men. We will assess the feasibility of using data from cell phones to obtain behavioral network data about interactions and communication between members of social networks. Using an existing computer software program that interfaces with users' cell phones, we will collect two types of data: (1) cell phone call and text message activity to ascertain communication frequency and patterns within networks, and (2) global positioning system (GPS) locations from cell phones to ascertain geographical proximity and frequency of face to face interactions between network members. We will enroll 120 emerging adult heterosexual men ages 18-25 from three social networks. Trained outreach workers will recruit three index participants using a venue-based sampling design targeting locations (e.g., bars, parks/basketball courts, barber shops) in the three highest risk areas (e.g., high STD rates, high poverty) within the target city. Close friends of the index will be recruited in the study. Subsequent waves of recruitment will enroll network members of participants from the previous wave to form three sociocentric networks. All participants will complete a thorough social network assessment where they provide a complete roster of their friendship network. In addition, participants will report their overall levels of sexual risk behavor (including condom use, concurrency, number of partners) and substance use (including alcohol, marijuana, and hard drug use) and we will monitor their cell phone activity, text messaging activity, and GPS locations for 6 months. This study will assess the feasibility and utility of usig cell phone data to obtain behavioral information about networks.